tipc: Improve handling of invalid link tolerance values

Enhances TIPC link code to ignore an invalid link tolerance value
contained in an incoming LINK_PROTOCOL message, rather than
processing the value and potentially causing a divide-by-zero error.

Also add a compile-time check that catches attempts to redefine
TIPC's minimum link tolerance value in a manner that might result
in the same divide-by-zero error at run-time.

Signed-off-by: Allan Stephens <Allan.Stephens@windriver.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>